Like this:

I’m going to be serious for a minute. These Yahoos of VanillaISIS say the constitution doesn’t allow for public lands? Yes, it does.

The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States; and nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States, or of any particular State.

The Oregon Territory was obtained via treaty with England (we got the Pacific NW, England got British Columbia). It began as public land and those portions not sold or turned over to the state of Oregon remain public land.